One example of a communication system for transmitting and receiving signals is a radio frequency identification (RFID) system. RFID is technology that automatically recognizes data stored in a tag, a label, or a card with a microchip using radio frequency by a reader.
RFID technology causes an RFID reader to read an RFID tag information. In a case of a passive RFID tag, there is no a need for a separate power supply, such as a battery, outside of the RFID tag for driving the RFID tag. An external RF continuous wave (RF CW) should be continuously supplied to the RFID tag such that the RFID tag may produce a power source to be driven thereby. However, when a passive RFID reader reads a signal generated from the RFID tag, it should continuously generate and transfer an RF CW signal to the RFID tag. In this case, since the transmitting signal is mixed with a signal generated by the RFID tag being transferred to the RFID reader (i.e., the mixed signal is received by the RFID reader), it is difficult to identify an RFID tag signal. In a case where only one antenna is used, because an intensity of a transmitting signal is much larger than that of a receiving signal in the RFID reader, leakage of a transmitting signal from a transmitting unit to a receiving unit cannot be isolated. The leakage of the transmitting signal to the receiving unit deteriorates the performance of the receiving signal.
FIG. 1 is a view illustrating a conventional apparatus 100 for isolating a leakage signal 150 of a transmitting signal from a signal input to a receiving unit 130 using an antenna 140 transmitting and receiving an RF signal, a transmitting unit 120 converting the signal into an RF transmitting signal TX1, a receiving unit 130 receiving and converting an output of port 2 into a baseband signal, and a circulator 110. The circulator 110 has a characteristic wherein it transfers a signal from port 1 to port 2, from port 2 to port 3, and from port 3 to port 1, but does not transfer the signal from port 2 to port 1, from port 1 to port 3, and from port 3 to port 2, which are directions opposite to correct operation. That is why the circulator 110 has non-reciprocity due to a strong magnetic field. The apparatus 100 may isolate a signal 150 leaked to the receiving unit 130 from the transmitting unit 120 using the non-reciprocity. In this manner, an isolation performance of approximately 25 dB may be obtained. However, when a frequency less than 1 GHz is used in the apparatus 100, for example, when a used frequency ranges from 860 MHz to 960 MHz, the apparatus 100 has a demerit that a circulator is very large and its cost is high.
FIG. 2 is a view illustrating a conventional apparatus 200 for isolating a leakage signal of a transmitting signal output from a transmitting unit 120 from a signal input to a receiving unit 130 using one antenna 140 transmitting and receiving an RF signal and a directional coupler 210. The directional coupler 210 of FIG. 2 has a signal through path in port 1→port 2 direction, a signal isolation path in port 1→port 3 direction, and a signal coupling path by coupling in port 1→port 4 direction. Further, each port has reciprocity with respect to remaining ports. The relationship between port 1 and port 3 becomes an isolation path to isolate a transfer of a transmitting signal TX1 of the transmitting unit 120 to port 3 connected to the receiving unit 130 but a part of the transmitting signal TX1 is leaked to become a leakage signal TX3, which is output to the receiving unit 130. In this case, an isolation performance of approximately 25 dB may be obtained.
As described above, the two apparatuses 100 and 200 are generally used in an RFID system to divide a transmitting signal TX1 and a receiving signal RX2. In the two cases, a leakage of a transmitting signal from a transmitting unit to a receiving unit cannot be completely isolated. The larger an intensity of the transmitting signal is, the larger an intensity of the leakage signal is.
FIG. 3 is a view illustrating an apparatus 300 for improving an isolation performance of a transmitting signal using one antenna 140, two directional couplers 210, a balanced oscillator 320, and a Wilkinson power combiner 330. The balanced oscillator 320 provide a signal having the same phase as that of a transmitting signal to one directional coupler 210 and a differential signal having a 180° phase difference from the transmitting signal to another directional coupler 210. The differential signal is canceled out in the Wilkinson power combiner 330 to remove the transmitting signal received through the antenna 140, with the result that only a desired receiving signal can be separated. However, the apparatus 300 uses a half of the transmitting signal for transmission and a remaining half thereof to cancel a signal leaked to the receiving unit 130. Consequently, transmission power consumption occurs.
FIG. 4 is a view illustrating a circuit for making a signal with a 180° phase difference from a transmitting signal using a vector modulator 430 to remove the transmitting signal. A part of the transmitting signal 471 is transferred to the vector modulator 430 through a first directional coupler 210 connected to the transmitting unit 120. The vector modulator 430 generates a cancellation signal 490 with the same magnitude as a magnitude and a phase different from a phase of the transmitting signal leaked from the circulator 110, and provides the cancellation signal 490 to the second directional coupler 210. Upon reception of the cancellation signal 490, the second directional coupler 210 sums the cancellation signal 490, the receiving signal 460 received through an antenna 140, and the leaked transmitting signal 450, thereby removing the leaked transmitting signal 450. In this manner, since a cancellation signal 490 with the same magnitude as a magnitude of the leaked transmitting signal 450 and a phase different from a phase of the leaked transmitting signal 450 is generated, a vector modulator 430 should be implemented correctly, which is difficult and complicated.